With the proliferation of light-emitting-diode (LED) lamps, as well as other types of lamps, there are many applications which include dimming the lamps. For example, it is often desirable for LED lamps in residential and commercial applications to be dimmable. In some cases, it may be desirable for LED lamps to have the capability to change intensity when used in instrumentation, user interface displays, and other information-related applications. Further, display screens for information or entertainment applications make use of LED lamps that dim and/or change colors.
Many dimmable LED lamp applications have a desired dimming range that includes less than 5% brightness. However, common LED drive circuits (such as switch-mode power supplies (SMPS), for example) may have a narrow output dimming range that stops at greater than 10% brightness, for example, when coupled to a dimmer with a narrow conduction angle. This means that dimming at the lowest (least bright) end of the dimming range may not be progressive, but may have abrupt steps in intensity, including turning off the LED at around 10% of the dimming range, for example. In many applications, this may be an undesirable dimming performance.
Generally, problems with dimming at the lower levels of the dimming range occur when the input current to an LED driver is reduced, causing unintended turn off of one or more driver components. For example, a reduced input current may result in an insufficient holding current to the one or more components, causing an early turn off of the components. This can result in a narrow dimming range, where the lower end of the dimming range is cut off at or above the 10% dimming level, and can also cause flickering issues with an LED lamp.